Isosorbide is a bicyclic diol, that is easily available from biological feedstock. Glucose can be hydrogenated to sorbitol. The latter, in turn, can be subjected to double dehydration so as to yield isosorbide. Its double hydroxyl function would make isosorbide of interest as a building block for polymerization. However, the making of polymers of suitable properties is hampered by the molecule's stereochemistry, since the two hydroxyl groups are directed to different sides of the molecule's plane. I.e., the up to now more easily obtainable isosorbide is unsymmetrical with one endo-hydroxyl group and one exo-hydroxyl group, resulting in asymmetrical reactivity and amorphous polymers (due to the lack of symmetry). Its epimer isomannide, which has two endo-hydroxyl groups, has proven to be unfavorable for polymerization due to low reactivity and low linearity. On the other hand, the epimer isoidide has two exo-hydroxyl groups, and has been viewed as far better suited for use as a building block for polymerization than either isosorbide or isomannide.
Isosorbide, which is the natural epimer, is capable of being converted into isomannide and isoidide, according to the following reaction scheme:

Isoidide as just mentioned is known as a building block for polymers. Examples include polyesters made by polycondensation of isoidide and a dicarboxylic acid or anhydride, and polycarbonates made by reaction with a bifunctional carboxyl compound such as phosgene. Isoidide is useful in other polymerizations wherein conventionally other diols are used. E.g., bisglycidyl ethers of isoidide can be used as a substitute for bisphenol-A in epoxy resins.
It is desired to be able to produce isoidide, whilst maintaining the original biomolecule as a feedstock. A suitable, economical method for commercial scale production, to date, does not exist.
As background art, reference is made to L. W. Wright, J. D. Brandner, J. Org. Chem., 1964, 29 (10), pp 2979-2982. Herein it is disclosed to induce epimerization by means of Ni catalysis, using nickel supported on diatomaceous earth. The reaction is conducted under relatively severe conditions, such as a temperature of 220° C. to 240° C. at a pressure of 150 atmosphere. The reaction reaches a steady state after two hours, with an equilibrium mixture containing isoidide (57%), isosorbide (36%) and isomannide (7%). Comparable results were obtained when starting from isoidide car isomannide. Increasing the pH to 10-11 was found to have an accelerating effect, as well as increasing the temperature and nickel catalyst concentration. A similar disclosure is to be found in U.S. Pat. No. 3,023,223.
A desire in the art is to find an effective method to obtain isoidide in good yields, yet allowing more moderate reaction conditions. An additional challenge in providing such a method, is the need to avoid side-reactions, which lead to undesirable mass loss, e.g. as a result of hydrodeoxygenation.
Another background reference is C. Montassier et al., J. Mol. Catalysis, 70, 1991, p 65. This document generally concerns conversion of polyols into furanic derivatives using bimetallic catalysts. An incidental reference is made to epimerization of isosorbide into isoidide over an SiO2 supported Ru catalyst. The disclosure does not, however, enable this epimerization. In fact, it can be derived from FIG. 6 in the reference, that after a reaction time of as long as six hours, a thermodynamic equilibrium has not been reached.
A further background reference is EP 1 647 540. Therein the synthesis is described of L-iditol starting from sorbitol. In a first step sorbitol is converted by fermentation into L-sorbose, which is subsequently hydrogenated into a mixture of D-sorbitol and L-iditol. This mixture is then converted into a mixture of L-iditol and L-sorbose. After separation from the L-sorbose, the L-iditol can be converted into isoidide. Thus, sorbitol is converted into isoidide in a four-step reaction, in a yield of about 50%. It would be desired to provide a more direct process, involving fewer steps, and providing a higher yield.
Hence, a need exists in the art to provide an effective method for the epimerization of isosorbide into isoidide, and catalysts therefore.